1. Field of the Invention
This invention relates to magnetic tape recording and reproducing arrangements, and more particularly to such arrangements including a rotary drum, helical scan digital video tape recorder (VTR).
2. Description of the Prior Art
Although the mechanical and timing tolerances are severe, it is now commonplace for a VTR to achieve excellent reproduction from a magnetic tape so long as the reproduction is in the normal mode, that is, at normal forward speed, that is at the same speed and in the same direction as was used for recording. It is, however, more difficult to achieve acceptably good reproduction at different speeds. Reproduction at different speeds is sometimes collectively referred to as stunt mode reproduction and includes the slow shuttle mode which generally comprises reproduction in both forward and reverse directions at speeds lower than the normal forward speed and also still-picture reproduction, and the fast shuttle mode which generally comprises reproduction in both forward and reverse directions at speeds higher than the normal forward speed; although the speed boundaries between these modes are variable and may even overlap. There are several problems with reproduction in stunt modes, but the basic problem is that the strict scanning correlation between reproducing heads and recorded tracks to be reproduced is lost. For example, in the fast shuttle mode a given reproducing head instead of scanning a single recording track from end to end may cross many tracks in a single scan across the magnetic tape. This means that the data reproduced from the tape is imperfect to a greater or lesser extent, and in consequence the quality of the reproduced television picture is degraded.
To counteract this it has been proposed to use so-called dynamic tracking. In this case each reproducing head is mounted on a electromechanical transducer, such as a bi-morph leaf made of piezoceramic material. The output of the reproducing head is sensed and a feedback loop applies potentials to the transducer to keep the head output at a maximum, in other words to cause the head to trace the recorded track accurately.
While dynamic tracking can give good results, it is not always convenient to use, it is somewhat complicated, and it is not very suitable for use with multiple-head head assemblies.